Photon Pharmacology of an Iron-sulphur Cluster Nitrosyl Compound Acting on Smooth Muscle

1. The mechanisms of action on smooth muscle of the iron-sulphur cluster nitrosyl compound, heptanitrosyl-tri-mu 3-thioxotetraferrate (1-), (RBS), a photosensitive nitric oxide donor, have been investigated in the guinea-pig taenia caeci (coli) in vitro. 2. After exposure to RBS (50 microM) for 30 min, and subsequent washout, a sustained contraction was recorded in the absence of light to either the agonist carbachol (50 microM) or a depolarizing concentration of KCl (23.5 mM). Photon irradiation (> 400 nm) caused a prompt relaxation of precontracted RBS-treated muscle, the magnitude of which depended upon the intensity (1.1 x 10(3) to 1.1 x 10(5) lux), duration (30 s to 20 min) and wavelength (400 to 800 nm), of the incident illumination. 3. Repeated periods of illumination at 1.1 x 10(4) lux produced a reversible relaxation of both carbachol and KCl-evoked tone in muscle pretreated with RBS (50 microM). These photorelaxations were reproducible at 10 min intervals for several hours with a maximal relaxation amounting to 80 to 90% that of the tone produced by carbachol (50 microM). 4. The nitric oxide synthase inhibitor, NG-nitro-L-arginine (60 microM), caused no inhibition of the photon-induced relaxation of RBS-treated muscle. In contrast, N-methylhydroxylamine (2 mM), L-cysteine (10 mM), DL-dithiothreitol (2 mM), methylene blue (30 microM), and haemoglobin (20 microM), all reversibly but significantly inhibited (P < 0.001) the photorelaxation response. However, neither the aminothiol N-acetyl-L-cysteine (10 mM) nor the tripeptide glutathione (10 mM) blocked the RBS-induced photorelaxation. 5 The photolytic cleavage of RBS depended on the intensity and duration of illumination; it was accompanied by a corresponding decrease in absorbance and by the liberation of NO as measured by the Griess diazo reaction with sulphanilic acid. L-Cysteine (10 mM) prevented the decrease in absorbance and the photolytic liberation of NO.6 It is concluded that (i) sequestered or bound RBS, when photon-activated, liberates NO by a process which can be controlled by the wavelength, intensity and duration of the incident light, (ii) the photon-released NO rapidly relaxes the smooth muscle cells of the taenia coli primarily via cyclic GMP-dependent pathways which can be blocked by use of appropriate inhibitors, and (iii) the RBS-induced photorelaxation effect does not involve the activation of NO synthase. RBS is therefore a valuable photosensitive NO donor for establishing the functional and pharmacological significance of NO.